The secretory cells produce a protein-rich mucus that is conveyed along the oviduct to the uterus by the cilia. This ciliary-mucus escalator maintains a healthy epithelium, moves the cumulus-oocyte complex toward the uterus, and may provide directional cues for swimming sperm. Movement of the cumulus-oocyte complex slows at the ampullary-isthmus junction, where fertilization normally takes place. This appears to be due in part to thick mucus produced by the isthmus and increased tone of the muscularis of the isthmus. The composition of oviductal secretions is complex and includes growth factors, enzymes, and oviduct-specific glycoproteins. Note that the clinical process of in vitro fertilization has shown that secretions of the oviduct are not absolutely necessary for fertility. However, normal oviductal function is absolutely required for both fertilization and implantation after in vivo insemination (i.e., natural sexual intercourse). Normal oviductal function also minimizes the risk of ectopic implantation and ectopic pregnancy, which occurs most often within the oviduct.
Hormonal Regulation During the Menstrual Cycle
In general, estrogen secreted during the follicular phase increases epithelial cell size and height in the endosalpinx. Estrogen increases blood flow to the lamina propria of the oviducts, promotes production of oviduct-specific glycoproteins (whose functions are poorly understood), and increases ciliogenesis throughout the oviduct. Estrogen promotes secretion of thick mucus in the isthmus and increases the tone of the muscularis of the isthmus, thereby keeping the cumulus-oocyte complex at the ampullary-isthmus junction for fertilization. High progesterone, along with estrogen, during the early luteal to midluteal phase decreases epithelial cell size and function. Progesterone promotes deciliation. It also decreases the secretion of thick mucus and relaxes the tone in the isthmus. In addition it should be noted that oviductal epithelial cells express the LH receptor, which may synergize with estrogen to optimize oviductal function during the periovulatory period.
The Uterus
The uterus is a single organ that sits in the midline of the pelvic cavity between the bladder and the rectum. The mucosa of the uterus is called the endometrium, the three-layered thick muscularis is called the myometrium, and the outer connective tissue and serosa are called the perimetrium. The parts of the uterus are (1) the fundus, which is the portion that rises superiorly from the entrance of the oviducts, (2) the body of the uterus, which makes up most of the uterus, (3) the isthmus, a short narrowed part of the body at its inferior end, and (4) the cervix, which extends into the vagina (see Figs. 44.13 and 43.25). Because the cervical mucosa is distinct from the rest of the uterus and does not undergo the process of menstruation, it will be discussed separately later.
The established functions of the uterus are all related to fertilization and pregnancy (discussed later). The main functions of the uterus are to: 1.
assist movement of sperm from the vagina to the oviducts 2.
provide a suitable site for attachment and implantation of the blastocyst, including a thick nutrient-rich stroma 3.